Thüringer Landessternwarte Tautenburg, Sternwarte 5,
2 Instituto de Astrofísica de Canarias (IAC), 38200 La Laguna, Tenerife, Spain
3 Departamento de Astrofísica, Universidad de La Laguna (ULL), 38205 La Laguna, Tenerife, Spain
4 Max-Planck-Institut für Extraterrestrische Physik, Giessenbachstraße, 85748 Garching, Germany
5 University of Rochester, Department of Physics and Astronomy, Rochester, NY 14627-0171, USA
6 Florida Institute of Technology, Melbourne, FL 32901, USA
7 Department of Physics and Astronomy, Clemson University, Clemson, SC 29634, USA
8 Department of Physics and Astronomy, Dickinson College, Carlisle, PA 17013, USA
9 Universe Cluster, Technische Universität München, Boltzmannstraße 2, 85748 Garching, Germany
10 Dark Cosmology Centre, Niels Bohr Institute, Univ. of Copenhagen, Juliane Maries Vej 30, 2100 Købnhaven, Denmark
11 INAF-IASF Bologna, Area della Ricerca CNR, via Gobetti 101, 40129 Bologna, Italy
12 Centre for Astrophysics and Cosmology, Science Institute, University of Iceland, Dunhagi 5, 107 Reykjavík, Iceland
13 American River College, Physics Dpt., 4700 College Oak Drive, Sacramento, CA 95841, USA
14 Instituto de Astrofísica de Andalucía (IAA-CSIC), Glorieta de la Astronomía s/n, 18.008 Granada, Spain
15 Institute of Experimental and Applied Physics, Czech Technical University in Prague, Horská 3a/22, 12800 Prague, Czech Republic
16 INAF – Osservatorio Astrofisico di Arcetri, Largo Fermi 5, 50125 Firenze, Italy
17 Institute of Astronomy, University of Cambridge, Madingley Road CB3 0HA, Cambridge, UK
18 Universitá degli Studi di Milano-Bicocca, Piazza della Scienza 2, 20126 Milano, Italy
19 Scuola Normale Superiore di Pisa – Piazza dei Cavalieri 7, 56126 Pisa, Italy
20 INAF – Osservatorio Astronomico di Trieste, via G.B. Tiepolo 11, 34143 Trieste, Italy
Received: 6 May 2011
Accepted: 5 June 2012
Context. Gamma-ray bursts (GRBs) can provide information about star formation at high redshifts. Even in the absence of a bright optical/near-infrared/radio afterglow, the high detection rate of X-ray afterglows by Swift/XRT and its localization precision of 2–3 arcsec facilitates the identification and the study of GRB host galaxies.
Aims. We focus on the search for the host galaxies of 17 bursts with arcsec-sized XRT error circles but no detected long-wavelength afterglow, in spite of their deep and rapid follow-up observations. Three of these events can also be classified as truly dark bursts, i.e., the observed upper limit on the optical flux of the afterglow was less than expected based on the measured X-ray flux. Our goals are to identify the GRB host galaxy candidates and characterize their phenomenological parameters.
Methods. Our study is based on deep RC and Ks-band observations performed with FORS1, FORS2, VIMOS, ISAAC, and HAWK-I at the ESO/VLT, partly supported by observations with the seven-channel imager GROND at the 2.2-m telescope on La Silla, and supplemented by observations with NEWFIRM at the 4-m telescope on Kitt Peak. To be conservative, we searched for host galaxy candidates within an area of twice the radius of each associated 90% c.l. Swift/XRT error circle.
Results. For 15 of the 17 bursts, we find at least one galaxy within the searching area, and in the remaining two cases only a deep upper limit to RC and Ks can be provided. In seven cases, we discover extremely red objects in the error circles, at least four of which might be dust-enshrouded galaxies. The most remarkable case is the host of GRB 080207, which has a color of (RC − Ks)AB ~ 4.7 mag, and is one of the reddest galaxies ever associated with a GRB. As a by-product of our study we identify the optical afterglow of GRB 070517.
Conclusions. Only a minority of optically dim afterglows are due to Lyman dropout (≲ 1/3). Extinction by dust in the host galaxies might explain all other events. Thereby, a seemingly non-negligible fraction of these hosts are globally dust-enshrouded, extremely red galaxies. This suggests that at least a fraction of GRB afterglows trace a subpopulation of massive starburst galaxies, which are markedly different from the main body of the GRB host galaxy population, namely the blue, subluminous, compact galaxies.
Key words: gamma-ray burst: general / galaxies: high-redshift
Based on observations collected at the Very Large Telescope of the European Southern Observatory, Paranal, Chile (ESO programmes 381.A-0647, 383.A-0399, 384.A-0414; PI: S. Klose; 081.D-0739, PI: A. Rossi, and 086.A-0533, PI: T. Krühler), GROND (PI: J. Greiner), and the Kitt Peak National Observatory (Program ID 2008B-0070; PI: A. C. Updike). Other observations are obtained from the ESO/ST-ECF Science Archive Facility.
Appendix A is available in electronic form at http://www.aanda.org
© ESO, 2012